Charge pump with reduced current mismatch

ABSTRACT

Charge pump circuitry is provided that is insensitive to charge sharing and current mismatch effects. The charge pump circuitry has an output node at which a charge pump output voltage is provided. A first current source charges the output node to increase the output voltage or a second current source discharge the output node to decrease the output voltage. The charge pump circuitry uses a unit-gain op-amp circuit to prevent charge sharing effects from affecting the output voltage when switching between discharging and charging operations. A low-pass filter is used to reduce feedback noise on the output node. A replica feedback circuit prevents current mismatch between the currents produced by the first and second current sources. The first and second current sources may be formed using programmable transistors that are adjusted by static control signals provided by programmable elements to further minimize current mismatch.

This application is a continuation of patent application Ser. No.11/142,880, filed May 31, 2005, now U.S. Pat. No. 7,385,429 which ishereby incorporated by reference herein in its entirety.

BACKGROUND

This invention relates to charge pump circuits, and more particularly,to charge pump circuits having current matching, non-startup prevention,and programmable current steering capabilities.

Charge pumps are used in circuits such as phase-locked loops in clockand data recovery circuits. Accurate charge pump behavior is importantto ensure optimum performance of the phase-locked loop.

Charge sharing effects and current mismatch effects adversely affectcharge pump performance. These effects reduce the accuracy ofconventional charge pumps and the circuits in which these conventionalcharge pumps operate.

SUMMARY

In accordance with the present invention, charge pump circuitry isprovided that is insensitive to charge sharing and current mismatcheffects.

The charge pump circuitry has a charge pump core and replica feedbackcircuitry. The charge pump core has inputs that receive charge pumpcontrol signals and an output node at which a charge pump output voltageis provided in response to the charge pump control signals.

The charge pump core has first and second current sources. The firstcurrent source is coupled between a positive power supply voltage andthe output node. The second current source is coupled between the outputnode and a ground power supply voltage. The charge pump core has twoparallel circuit branches that are coupled by a unit-gain operationalamplifier. The parallel circuit branches each include transistors suchas metal-oxide-semiconductor transistors having gates controlled by thecharge pump control signals. By turning on and off the transistors inthe charge pump core, the charge pump control signals control whetherthe first current source is charging the output node or whether thesecond current source is discharging the output node.

When the output node is charged, the output voltage on the output noderises toward a high voltage. When the output node is discharged, theoutput voltage is lowered toward a low voltage. The unit-gain operationamplifier couples the first and second branches and ensures that theoutput node does not suffer from charge sharing effects as the controlsignals switch the charge pump circuitry between discharging andcharging modes.

The replica feedback circuitry provides feedback signals that ensurethat the currents produced by the first and second current sources donot become mismatched. Second-order current mismatch effects are alsominimized by using programmable transistors in the first and secondcurrent sources. The strengths of the programmable transistors can beadjusted by loading appropriate configuration data into programmableelements on the integrated circuit on which the charge pump circuitry isimplemented. The programmable elements produce corresponding staticcontrol signals that are applied to respective transistor stages in theprogrammable transistors. The static control signals connect anddisconnect portions of the programmable transistors to adjust theirstrengths. By balancing the strengths of the first and second currentsources, current mismatch is reduced.

The charge pump circuitry contains a feedback path from the output nodethrough the first current source and back to the output node. To reducenoise coupling, particularly at high frequencies, low-pass filtercircuitry is interposed in the feedback path.

The replica feedback circuitry contains an operational amplifier in atwo-stage configuration. The operational amplifier contains ametal-oxide-semiconductor transistor characterized by a thresholdvoltage. The gate of the transistor is connected to the output node. Toprevent the voltage of the output node from becoming stuck when theoutput voltage is lower than the threshold voltage, a static currentsource is used to contribute a small current to the total currentproduced by the first current source. Even if the voltage on the outputnode is low and the operational amplifier in the replica feedbackcircuitry is not active because the threshold voltage of the transistorhas not been exceeded, the static current source will charge the outputnode and raise its voltage.

Further features of the invention, its nature and various advantageswill be more apparent from the accompanying drawings and the followingdetailed description of the preferred embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of an illustrative programmable logic device inaccordance with the present invention.

FIG. 2 is a diagram of illustrative clock and data recovery circuitry inan integrated circuit such as a programmable logic device integratedcircuit in accordance with the present invention.

FIG. 3 is a diagram of a illustrative phase-locked loop using a chargepump in accordance with the present invention.

FIG. 4 is a diagram of a charge pump circuit of the type that is subjectto the adverse effects of charge sharing.

FIG. 5 is a diagram of charge pump circuitry that addresses chargesharing effects.

FIG. 6 is a diagram of charge pump circuitry that addresses currentmatching effects.

FIG. 7 is a diagram of illustrative charge pump circuitry that addressescharge sharing and current matching effects in accordance with thepresent invention.

FIGS. 8 and 9 are diagrams of illustrative programmable transistors thatmay be used in the charge pump circuitry of FIG. 7 in accordance withthe present invention.

DETAILED DESCRIPTION

The present invention relates to charge pumps and to methods for usingcharge pumps. The charge pump circuitry of the present invention may beused in integrated circuits such as programmable logic device integratedcircuits.

Programmable logic devices are integrated circuits that can beconfigured by a user to perform custom logic functions. Programmablelogic devices are configured (“programmed”) by loading configurationdata into the device. The configuration data selectively turns on andoff components of the device's circuitry and thereby customizes thelogic on the device. Programmable logic devices with non-volatileconfiguration memory retain their configuration data when power isremoved. Programmable logic devices with volatile configuration data aretypically loaded with configuration data on system power-up using anancillary circuit called a configuration device. The configurationdevice may store the configuration data using non-volatile memory.Programmable logic devices with non-volatile memory can also beconfigured after installation in a system (so-called in-systemprogramming).

The present invention may be used in the context of any suitableintegrated circuit, but is described in the context of programmableintegrated circuits such as programmable logic devices for clarity.

Integrated circuits such as programmable logic devices often use chargepumps. For example, integrated circuits often have communicationscircuitry for supporting data communications links with other integratedcircuits. The communications circuitry may include clock and datarecovery circuitry that extracts an embedded clock signal and data fromdata signals received from another integrated circuit. Clock-and-datarecovery circuits use phase-locked loops. A charge pump is part of aphase-locked loop circuit.

An illustrative programmable logic device 10 of the type that maycontain a charge pump is shown in FIG. 1.

Programmable logic device 10 may have input-output circuitry 12 fordriving signals off of device 10 and for receiving signals from otherdevices via input-output pins 14. Pins 14 may be any suitable types ofpins or solder bumps for making electrical connections between theinternal circuitry of device 10 and external packaging. Some of the pins14 may be used for high-speed communications signals. Other pins may beused to provide power supply voltages to the device 10 or may be usedfor DC or low-frequency signals.

Interconnection resources 16 such as global and local vertical andhorizontal conductive lines and busses may be used to route signals ondevice 10. The remainder of the circuitry 18 on device 10 includesblocks of programmable logic, memory blocks, regions of digital signalprocessing circuitry, processors, hardwired circuits for supportingcomplex communications and arithmetic functions, etc. The programmablelogic in circuitry 18 may include combinational and sequential logiccircuitry including logic gates, multiplexers, switches, memory blocks,look-up-tables, logic arrays, etc. These illustrative components are notmutually exclusive. For example, look-up tables and other componentsthat include logic gates and switching circuitry can be formed usingmultiplexers.

Some of the logic of programmable logic device 10 is fixed (hardwired).The programmable logic in device 10 includes components that may beconfigured so that device 10 performs a desired custom logic function.The programmable logic in programmable logic device 10 may be based onany suitable programmable technology. With one suitable approach,configuration data (also called programming data) may be loaded intoprogrammable elements in the programmable logic device 10 using pins 14and input/output circuitry 12. During normal operation of device 10, theprogrammable elements (also sometimes called configuration bits orconfiguration memory) each provide a static control output signal thatcontrols the state of an associated logic component in the programmablelogic of circuitry 18.

In a typical volatile memory arrangement, the programmable elements maybe random-access memory (RAM) cells that are loaded from an externalconfiguration device integrated circuit via certain pins 14 andappropriate portions of input/output circuitry 12. The loaded RAM cellsprovide static control signals that are applied to the terminals (e.g.,the gates) of circuit elements (e.g., metal-oxide-semiconductortransistors) in the programmable logic of circuitry 18 to control thoseelements (e.g., to turn certain transistors on or off) and therebyconfigure programmable logic device 10. Circuit elements in input/outputcircuitry 12 and interconnection resources 16 are also generallyconfigured by the RAM cell outputs as part of the programming process(e.g., to customize I/O and routing functions). The programmablecircuitry that is configured in input/output circuitry 12,interconnection resources 16, and circuitry 18 may be transistors suchas pass transistors or parts of multiplexers, look-up tables, logicarrays, AND, OR, NAND, and NOR logic gates, etc.

RAM-based programmable logic device technology is merely oneillustrative example of the type of technology that may be used toimplement programmable logic device 10. Other suitable programmablelogic device technologies that may be used for device 10 includeone-time programmable device arrangements such as those based onprogrammable logic elements made from electrically-configured fuses orelectrically-configured antifuses, programmable logic devices in whichelements 20 are formed from electrically-programmable read-only-memory(EPROM) or erasable-electrically-programmable read-only-memory (EEPROM)technology, programmable logic devices with programmable elements madefrom magnetic storage elements, programmable logic devices withprogrammable elements made from phase-change materials, mask-programmeddevices, etc. Illustrative programmable logic elements are shownschematically as elements 20 in FIG. 1.

The circuitry of device 10 may be organized using any suitablearchitecture. As an example, the logic of programmable logic device 10may be organized in a series of rows and columns of larger programmablelogic regions or areas each of which contains multiple smaller logicregions or areas (e.g., areas of logic based on look-up tables ormacrocells). These logic resources may be interconnected byinterconnection resources 16 such as associated vertical and horizontalinterconnection conductors. Interconnection conductors may includeglobal conductive lines that span substantially all of device 10,fractional lines such as half-lines or quarter lines that span part ofdevice 10, staggered lines of a particular length (e.g., sufficient tointerconnect several logic areas), smaller local lines that interconnectsmall logic regions in a given portion of device 10, or any othersuitable interconnection resource arrangement. If desired, the logic ofdevice 10 may be arranged in more hierarchical levels or layers in whichmultiple large areas are interconnected to form still larger portions oflogic. Still other device arrangements may use logic that is notarranged in rows and columns. Portions of device 10 (e.g., ininput/output circuitry 12 and elsewhere) may be hardwired forefficiency. As an example, hardwired wireless circuitry may be used toreceive wirelessly-transmitted configuration data. Other hardwiredcommunications circuitry and digital signal processing circuitry (e.g.,multipliers, adders, etc.) may also be used.

Programmable logic device 10 may transmit and receive data though itsI/O circuitry 12. For example, programmable logic device 10 may receivedata from another integrated circuit over a communications path 22, asshown in FIG. 2. Path 22 may be connected to the differential inputs ofa differential input buffer 24. The outputs of differential input buffer24 may be connected to a clock-and-data recovery circuit 26.

Communications link 22 of FIG. 2 may be a high-speed differential datalink in which differential data is conveyed over a pair of associatedsignal lines. The lines in link 22 may include conductive traces such aslines on one or more circuit boards, cables, system backplane busses,etc.

The integrated circuit from which the data on link 22 is transmitted mayhave a transmitter that serializes data and that embeds a clock into thesignal. The data with the embedded clock may be transmitted toprogrammable logic device 10 over link 22.

The embedded clock signal may be generated from a known reference clockthat is received at terminal 28. The reference clock may be generated byan oscillator on a circuit board on which device 10 is mounted. Theserial data may be encoded using any suitable format. For example, thedata may be non-return-to-zero (NZR) data.

The programmable logic device 10 may receive the differential datasignals from lines 22 using a differential input buffer 24. Input buffer24 strengthens and level shifts the differential data. At the output ofthe input buffer 24, the differential data signals have the appropriatehigh and low logic levels for processing by the digital circuitry ondevice 10.

The output signals from input buffer 24 are received by a clock and datarecovery circuit 26. Clock and data recovery circuit 26 also receivesthe reference clock at input 28.

The clock and data recovery circuit 26 may have a phase-locked loop thatlocks onto the reference clock. Once the clock and data recovery circuit26 has acquired the reference clock, the clock and data recovery circuitmay be used to extract the embedded clock. The recovered embedded clockmay be used to recover the data from the data stream.

The clock and data recovery circuit 26 may pass the recovered data andrecovered clock to a deserializer 30 over data path 32 and recoveredclock path 34. The deserializer 30 may use the recovered clock toconvert the recovered data into a parallel data format. The paralleldata may be provided to other circuitry on programmable logic device 10on parallel output lines 35.

An illustrative phase-locked-loop 36 that may be used in circuits suchas clock and data recovery circuit 26 of FIG. 2 is shown in FIG. 3.Phase-locked loop 36 is formed by phase/frequency detector 38, chargepump 40, loop filter 42, voltage controlled oscillator 44, and frequencydivider 46. In this example, phase-locked loop 36 is being used to lockto the reference clock on line 28 and provides the recovered clock online 34.

Phase/frequency detector 38 compares the reference clock provided toreference clock input 28 to a feedback clock FBCLK from thevoltage-controlled oscillator 44 at input 54. The phase/frequencydetector 38 provides output signals UP and DN on its outputs 48. Whenthe reference clock is ahead of the FBCLK, the control signals UP and DNhave a first state. When the reference clock is behind FBCLK, the UP andDN signals are placed in a second state. The phase/frequency detectoroutputs UP and DN therefore act as control signals for the rest of thephase-locked loop. These control signals are used to adjust thefrequency of the feedback signal produced by the voltage-controlledoscillator 44 so that it matches the reference clock.

The output signals UP and DN from the phase/frequency detector 38 areprovided to the inputs of charge pump 40. The charge pump 40 produces ananalog output signal V_(CTRL) that is proportional to the incomingcontrol signals UP and DN. This signal is provided to loop filter 42over path 50. Loop filter 50 filters this signal to remove undesirablefrequency components and provides the filtered version of the signal tovoltage-controlled oscillator 44 as an input over line 52.Voltage-controlled oscillator 44 produces an output signal whosefrequency is proportional to the voltage on line 52. The output signalfrom the voltage-controlled oscillator 44 may be fed back to input 54 ofthe phase/frequency detector 38 via paths 56, 58, and 60. An optionalfrequency divider 46 may be used in this feedback path if desired.

The performance of circuit 36 and other circuits that rely on chargepumps such as charge pump 40 is affected directly by the accuracy ofcharge pump 40. If charge pump 40 exhibits non-ideal behavior, theperformance of circuit 36 will suffer.

One source of problems in charge pump performance is the so-calledcharge sharing effect. A charge pump circuit 62 of the type that isstrongly influenced by charge sharing effects is shown in FIG. 4. Thecharge pump circuit 62 of FIG. 4 has two current sources 68 and 86.Current source 68 supplies a current of I_(UP) and current source 86supplies a current of I_(DN). For the purposes of this charge sharingdiscussion, assume I_(UP)=I_(DN). The current sources are connectedbetween a positive power supply node 64 at voltage V and a groundterminal 66 at ground.

The output of charge pump 62 is the voltage control signal V_(CTRL)supplied at line 78. Line 78 is connected to node 76, which lies betweenswitch 70 and switch 80. Switch 70 is controlled by the UP controlsignal applied to control input 74. Switch 80 is controlled by the DNcontrol signal applied to control input 82. When circuit 62 is used in aphase-locked loop such as loop 36 of FIG. 3, the UP and DN signals areproduced by the output of a phase/frequency detector such asphase/frequency detector 38.

In one mode, the UP and DN signals direct switch 70 to open and switch80 to close. Because switch 80 is closed, the current source 86 iselectrically connected to node 76 through a low-resistance path andremoves charge from node 76 so that voltage V_(CTRL) drops.

While switch 70 is open, charge is building at node 72. When it becomestime for the mode of operation of circuit 62 to change, the UP and DNsignals change state. In this situation, switch 70 is closed and switch80 is open. At the moment switch 70 is closed, the charge that has beenstored on node 72 is “shared” with the charge on the node 76, disruptingthe control voltage V_(CTRL) and thereby introducing error into theoperation of the charge pump circuit 62.

The charge pump circuit of the present invention introduces circuitry tocompensate for charge sharing effects. A portion of a charge pumpcircuit that compensates for charge sharing is shown in FIG. 5. Incharge pump circuitry 84, the switching functions of switches 70 and 80of FIG. 4 are performed by metal-oxide-semiconductor (MOS) transistors.Transistors 86 and 88 in the left-hand branch of circuitry 84, servefunctions similar to those of switches 70 and 80 of FIG. 4. Paralleltransistors 90 and 92 in the right-hand branch of circuitry 84 andop-amp 94 are used to compensate for charge sharing effects.

Transistors 88 and 92 are n-channel MOS (NMOS) transistors. Transistors86 and 90 are p-channel MOS (PMOS) transistors. Transistors 88 and 92are on when a “1” is applied to their gates. Transistors 86 and 90 areturned on when a “0” is applied to their gates.

Transistor 88 is driven by the DN signal applied to its gate. An UPsignal controls transistor 86. Transistors 90 and 92 are controlledrespectively by NOT UP and NOT DN signals (i.e., the complements ofsignals UP and DN used to control transistors 86 and 88).

Op-amp 94 is connected in a unit gain buffer configuration between nodes100 and 102. In this configuration, the gain of op-amp 94 is one and itsoutput 96 is always equal to its input 98.

The way in which charge sharing effects are compensated by thearrangement of FIG. 5 can be understood by an example. Consider thesituation in which UP=1, NOT UP=0, DN=1 and NOT DN=0. In this situation,PMOS transistor 86 is off and transistor 88 is on, so V_(CTRL) drops dueto the discharging effect produced by the current I_(DN) from source104. PMOS transistor 90 is on, so node N1 is at the same potential asnode 100 at the output of op-amp 94. The presence of op-amp 94 ensuresthat node 100 and therefore node N1 is equal to V_(CTRL). When the stateof the pump circuitry changes to UP=0, NOT UP=1, DN=0 and NOT DN=1,transistor 86 turns on and charges V_(CTRL) up. Because the voltage atN1 was maintained at V_(CTRL) prior to the change in mode through theoperation of op-amp 94, node N1 does not share charge with the V_(CTRL)node when the mode changes.

Another non-ideal charge pump characteristic that is addressed by thecharge pump circuitry of the present invention is the so-called currentmatching effect. For proper operation of a circuit such as circuit 84 ofFIG. 5, the current I_(DN) that is produced by source 104 should exactlymatch the current I_(UP) that is produced by source 106 for all inputsignals UP and DN and all output voltages V_(CTRL). If the currentsI_(DN) and I_(UP) are not always equal, the charge on node 102 andtherefore the voltage V_(CTRL) on node 102 will either rise faster orslower than expected, leading to non-linear behavior.

Consider, for example, the situation in which current sources 106 and104 are formed by MOS transistors. As the voltage V_(CTRL) changes dueto changes in the UP and DN control signals, the drain-to-source voltagedrop V_(DS) in these MOS transistors changes. In particular, as V_(CTRL)rises, V_(DS) for the I_(UP) source will drop and V_(DS) for the I_(DN)source will rise, causing I_(UP) to become smaller than I_(DN). As thisdemonstrates, the circuitry of the charge pump must be tolerant tochanges in drive signals and output voltage. If the circuitry of thecharge pump is too sensitive, the currents I_(DN) and I_(UP) will becomeunequal easily, which will make the performance of charge pump circuitry84 non-linear and inaccurate.

A conventional charge pump circuit 108 that contains features foraddressing the current matching problem is shown in FIG. 6. In circuitry108 of FIG. 6, control signals UP and DN are applied to the gates oftransistor M1 and transistor M4, respectively. Op-amp 110 drives thegates of transistors M2 and M6 and is connected in a so-called two-stageop-amp configuration (line 116 forms a negative feedback path). Becauseof the two-stage op-amp configuration, the voltage REF on node 112follows the voltage V_(CTRL) on node 114. This effectively couples theleft and right halves of circuit 108.

The charge pump circuit 108 of FIG. 6 operates in charging anddischarging modes.

In charging mode, UP=0 and DN=0. In this situation, PMOS transistors M1and M5 are on and the source voltages of transistors M2 and M6 are atidentical voltages. The drain of M2 is at V_(CTRL) and the drain oftransistor M6 is at voltage REF. Because of op-amp 110, the voltage REFand V_(CTRL) are equal, so the drains of M2 and M6 are at equalvoltages. As shown in FIG. 6, the gates of M2 and M6 are also connectedto the same potential. By symmetry, the current I₁ through transistor M2and the current I₂ through transistor M6 are equal. The current I₂ andI₃ are equal, because they lie in the same branch of the circuit. As aresult, I₁=I₂=I₃.

Current I₃ is produced by a current mirror circuit. This current mirroris formed from reference current source Ich and transistors M10 and M7.Transistor M7 lies in the right-hand branch 118 of circuit 108. Asimilar current mirror for the left-hand branch 120 of circuit 108 isformed from source Ich and transistors M10 and M3. The current Ich isproduced by a current mirror reference current source. The ratios of thestrengths of transistors M3, M7, and M10 set the ratios of the currentmirrors. The ratio of M3 to M10 determines the ratio of current I₄ toreference current Ich. The ratio of M7 to M10 determines the ratio ofcurrent I₃ to Ich. Because I₃ is produced by a current mirror, the valueof I₁=I₃ is also controlled by Ich and the M10/M7 ratio. Thus, the valueof current source I₁ is not a function of V_(CTRL) and is always equalto I₃, which is set by the reference current Ich.

In discharging mode, DN=1 and UP=1. In this situation, M3, M7, M4, M8,M9, M10, and source Ich form current mirror circuits. The ratio ofM3/M10 determines the ratio of I₄/Ich. The ratio M7/M10 determines theratio of I₃/Ich.

Transistor M1 is off during discharging, so I₁ is low. The value of I₂does not affect the performance of the circuit. Transistors M3 and M4are on and the magnitude of current I₄ is controlled by Ich and theratio of M10/M3. Transistors M7 and M3 are identical, so I₄ will matchI₃ (provided that M4 is on as it is during discharging), regardless ofthe magnitude of V_(CTRL).

The magnitude of V_(CTRL) during charging and discharging affects thesize of I₃. For example, at low V_(CTRL) values, the value of I₃ mightbe 10 microamps, whereas at higher V_(CTRL) values, the value of I₃might be 15 microamps. Although the magnitude of I₃ is not fixed as afunction of V_(CTRL), circuit 108 ensures that I₁=I₄ for all V_(CTRL).During charging mode when current I₁ charges node 114 and causesV_(CTRL) to rise, I₁ is set to I₃. Similarly, during discharging modewhen current I₄ discharges node 114 and causes V_(CTRL) to fall, I₄ isset to I₃. This ensures that I₁=I₄ for all V_(CTRL), thereby addressingthe charge pump current mismatch problem.

FIG. 7 shows a charge pump 40 in accordance with the present invention.Charge pump 40 of FIG. 7 may be used in phase-locked loops such as loop36 of FIG. 3 or in other suitable circuits. The charge pump 40 hasfeatures that address both charge sharing and current matching issues.

Charge pump 40 is powered by a positive power supply voltage V_(DD) anda ground power supply voltage V_(SS). Control signals UP, NOT UP, DN,and NOT DN are applied to charge pump 40 (e.g., from the output of aphase-frequency detector such as phase-frequency detector 38 of FIG. 3)at the gate terminals of transistors 120, 122, 124, and 126,respectively. In the example of FIG. 3, phase-frequency detector 38 isshown as producing UP and DN signals. When driving charge pump circuit40 of FIG. 7, the complements of these signals (NOT UP and NOT DN) areproduced by including inverters in the output stage of phase-frequencydetector 38. In general, the polarity of signals such as UP and DN isnot critical and can be configured in any suitable way by selective useof inverters and NMOS and PMOS transistors in the charge pump and/orexternal circuits such as phase-frequency detector 38.

In the example of FIG. 7, transistors 122 and 120 are PMOS transistorsand transistors 124 and 126 are NMOS transistors. When the gates oftransistors 120 and 122 are high, transistors 120 and 122 are off. Whenthe gates of transistors 120 and 122 are low, transistor 120 and 122 areon. Transistors 124 and 126 are turned off by low signals on their gatesand are turned on by high signals on their gates. Transistors 132, 134,136, and 138 are biased in an always on configuration and serve asisolation transistors. The gate-drain capacitance C_(GD) of thesetransistors will allow high frequency signals (e.g., high frequency NOTUP signals) to pass to charge pump output line 140.

The left-hand portion of charge pump 40 is called the charge pump coreand includes two parallel branches 128 and 130. Circuit branch 128includes transistors 122 and 124. Circuit branch 130 includestransistors 120 and 126. These two branches are coupled by an op-amp142. Op-amp 142 is connected in a unit gain buffer configuration betweennodes 144 and 146. In this configuration, the gain of op-amp 142 is oneand its output at node 146 is always equal to its input at node 144. Asdescribed in connection with circuit 84 of FIG. 5, this ensures thatnode 140 will not be subjected to charge sharing effects from node N1when pump 40 switches between discharging and charging modes.

The right-hand portion of charge pump 40 forms a replica feedbackcircuit. The replica feedback circuit provides feedback signals to thecharge pump core that minimize current mismatch between the Idn currentsource formed from transistor TB and the Iup current source formed usingtransistors 170 and 128. Transistors 148 and 150 in the replica feedbackcircuitry form an operational amplifier (op-amp) 158 having inputs 152and 154 and an output 156. The transistors of op-amp 158 are MOStransistors that are characterized by a threshold voltage Vth. Op-amp158 is connected in a two-stage configuration and couples the circuitryof branches 128 and 130 to circuit branch 160. The output 156 of op-amp158 provides feedback to the charge pump core and is coupled to thegates of PMOS transistors 170 and 174, which ties together the operationof these transistors in the same way as transistors M2 and M6 of FIG. 6.

Replica feedback circuit portion 160 includes four transistors 162, 164,166, and 168 that match the four-transistor arrangements of branches 128and 130 (e.g., by matching transistors 122, 132, 136, and 124 in branch128).

Circuitry 160 includes transistor TD, which forms a current mirror withtransistor TA and the Iref current source 176. Transistors TB and TCform current mirrors with source 176 and transistor TA. Op-amp 158causes the voltage at the REF node to follow the voltage V_(CTRL), sothat the condition in which Iup=Irp=Idn is achieved, as described inconnection with I₁, I₃, and I₄ of FIG. 6. This makes Iup=Idn to minimizecurrent mismatch. The replica feedback arrangement of circuit 40therefore ensures that the discharging current and the charging currentfor the charge pump core are the same, thereby overcoming potentialcurrent matching problems.

In discharging mode, DN is 1 and UP is 0. NOT DN is 0 and NOT UP is 1.In this mode, the discharging current Idn is controlled by Iref, becauseof the current mirror formed by transistors TA and TB. Note that thereis always an Idn current, so long as the DN control signal is high.

In charging mode, UP is 1, NOT UP is 0, DN is 0, and NOT DN is 1. Inthis situation, the charging current Iup for the charge pump core isIup=Irp (i.e., the charging current is tied to the reference current Irpof branch 160, as described in connection with currents I₁ and I₃ inFIG. 6).

Unlike the Idn current, the charging current Iup that is produced usingtransistor 170 can only be provided properly if the voltage V_(CTRL) onnode 144 is greater than the threshold voltage Vth of transistor 148.This is because the voltage on node 144 is provided to the gate oftransistor 148 via op-amp input 152. If V_(CTRL) is less than Vth, thecurrent contribution to Iup from transistor 170 will be zero. To ensurethat V_(CTRL) is greater than Vth of transistor 148, the Iup currentsource is provided with a small current contribution Istatic. Thecurrent Istatic is provided by the current mirror circuit formed fromcurrent source 178 and transistors 180 and 182. The current Iup is equalto the sum of Istatic and Imain. Istatic is static and is not controlledby feedback. The signal NOT UP is 0, so transistor 122 is on andconducts the current Istatic to node 144, thereby charging V_(CTRL) uppast Vth. Because V_(CTRL) is greater than Vth of transistor 148, theop-amp 158 works properly and provides feedback to transistor 170 in theIup current source. The Iup current therefore tracks Irp, which in turnis controlled by the current mirror formed by source 176 and transistorsTA and TD. To guarantee perfect current matching, the same amount ofcurrent (Istatic) is added to Irp using transistors 180 and 188.

As this discussion demonstrates, the current mirror circuitry of source178 and transistors 180 and 182 forms a current source that provides acurrent Istatic that ensures that the feedback path 184 of charge pump40 works properly at low values of V_(CTRL). As a result, the value ofV_(CTRL) does not become stuck at a particular voltage level insituations in which V_(CTRL) is less than Vth of transistor 148. The useof current mirror circuitry to provide Istatic is merely illustrative.Any suitable circuitry may be used in charge pump 40 to preventnon-startup problems from developing.

Path 186 represents a negative feedback path that is stable. Path 184,however, is a positive feedback path, and can serve as a potentialsource of noise feedback in pump 40. If noise appears on output 140,positive feedback through path 184 could potentially increase this noiselevel. Accordingly, charge pump 40 preferably includes filter circuitry190 interposed in the feedback path between node 144 and the Iup currentsource. Filter circuitry 190 may be based on any suitable filter circuitfor reducing noise feedback. In the illustrative example of FIG. 7,filter circuitry 190 includes a capacitor 192 and resistor 194 in alow-pass filter configuration. This configuration blocks high-frequencynoise components in the feedback path 184 (e.g., at frequencies of about10 MHz and higher in an illustrative circuit implementation) andprevents excessive noise from developing on line 140.

In typical implementations of charge pump 40 in an integrated circuit,layout asymmetries and other non-ideal factors may create a second-ordercurrent mismatch between Iup and Idn. This source of mismatch may becorrected by balancing the strengths of transistors 170 and TB.

Any suitable technique may be used to balance transistors 170 and TB.With one suitable approach, transistor 170 and/or transistor TB may beprogrammable, so that the relative strengths of transistors 170 and TBmay be adjusted. By adjusting the strengths of transistor 170 and/ortransistor TB during a setup balancing operation (e.g., at amanufacturer following testing), second order current mismatch effectscan be compensated.

An illustrative programmable transistor 170 is shown in FIG. 8. Anillustrative programmable transistor TB is shown in FIG. 9. As shown inFIGS. 8 and 9, the programmable transistors 170 and TB have multipleparallel controllable transistor stages. Each stage has a transistor 198with a gate connected to a respective switch 196. Switches 196 may beformed, for example, by NMOS and/or PMOS transistors controlled bystatic control signals from programmable elements 20. The programmableelements 20 are loaded with configuration data that causes their outputsto produce the signals desired to turn on and off appropriate switches196. This controls the associated stages of the programmable transistors170 and TB by connecting and disconnecting the gates of varioustransistors 198 from the circuit. By producing control signals withprogrammable elements 20, the gate signals PREFV and NREFV areselectively applied only to certain selected subsets of the transistors198 during operation. The other transistors are disconnected fromtransistor 170 (or TB) because their switches 196 are open. Theseprogrammable transistor adjustments are used to change the strengths oftransistors 170 and TB to eliminate second-order current mismatcheffects and thereby further minimize overall current mismatch in chargepump 40. Adjustments to the programmable transistors may be made at anysuitable time such as when a manufacturer performs fabrication andtesting operations in preparation for delivery of the programmable logicdevice to a customer.

The foregoing is merely illustrative of the principles of this inventionand various modifications can be made by those skilled in the artwithout departing from the scope and spirit of the invention.

1. A charge pump circuit comprising: a charge pump core having first andsecond current sources, an output node, and a positive power supplyvoltage line, wherein at least one of the current sources comprises aprogrammable transistor, wherein the programmable transistor has aplurality of parallel adjustable transistor stages each having asource-drain terminal directly connected to the positive power supplyvoltage line; replica feedback circuitry coupled to the charge pump corethat minimizes current mismatch between the first and second currentsources; and a low-pass filter connected between the charge pump coreand the replica feedback circuitry that reduces noise feedback at theoutput node.
 2. The charge pump circuit defined in claim 1 wherein thecharge pump core further comprises two circuit branches that are coupledby a unit-gain-amp.
 3. The charge pump circuit defined in claim 1wherein the output node is charged and discharged by the first andsecond current sources to produce an output voltage in response tocontrol signals.
 4. A charge pump circuit comprising: a charge pump corehaving a first current source, an output node, and a positive powersupply voltage line, wherein the first current source comprises aprogrammable transistor, wherein the first current source contains atleast one transistor having a gate that is part of a feedback path fromthe output node through the first current source back to the outputnode, and wherein the programmable transistor has a source-drainterminal directly connected to the positive power supply voltage line;and a low pass filter having a capacitor connected between the gate anda source of positive voltage and a resistor connected to the gate in thefeedback path.
 5. The charge pump circuit defined in claim 4 wherein thecharge pump core further comprises two circuit branches that are coupledby a unit-gain-amp.
 6. The charge pump circuit defined in claim 4,further comprising a second current source.
 7. The charge pump circuitdefined in claim 6 wherein the output node is charged and discharged bythe first and second current sources to produce an output voltage inresponse to control signals.
 8. The charge pump circuit defined in claim4 further comprising replica feedback circuitry coupled to the chargepump core to minimize current mismatch within the charge pump core.
 9. Acharge pump circuit comprising: first and second current sources,wherein at least one of the current sources is controlled by staticcontrol signals produced by programmable elements, wherein the at leastone current source comprises a programmable transistor, and wherein theprogrammable transistor has a source-drain terminal connected to apositive power supply voltage line; an output node, wherein the outputnode is either charged by the first current source or is discharged bythe second current source; a low-pass filter interposed in a feedbackpath from the output node through the first current source back to theoutput node to reduce noise feedback along the feedback path, whereinthe first current source contains at least one transistor having a gatethat is part of the feedback path; and a transistor that provides astatic current contribution to the at least one current source toprevent an output voltage on the output node from becoming stuck at avoltage level.
 10. The charge pump circuit defined in claim 9 whereinthe programmable transistor has a plurality of parallel transistorstages that are selectively switched into use in response to the staticcontrol signals produced by the programmable elements.
 11. The chargepump circuit defined in claim 9 wherein the output node is charged anddischarged in response to charge pump control signals.
 12. The chargepump circuit defined in claim 9 further comprising first and secondtransistors in a first circuit branch between the first and secondcurrent sources.
 13. The charge pump circuit defined in claim 12 furthercomprising third and fourth transistors in a second circuit branchbetween the first and second current sources.
 14. The charge pumpcircuit defined in claim 12 wherein the output node is located betweenthe first and second transistors in the first circuit branch.
 15. Thecharge pump circuit defined in claim 12 further comprising anoperational amplifier that electrically couples the first and secondbranches.
 16. The charge pump circuit defined in claim 9 furthercomprising an operational amplifier connected in a unit gainconfiguration between the output node and a circuit node located betweenfirst and second transistors disposed within the charge pump circuit.17. The charge pump circuit defined in claim 9 further comprising anoperational amplifier that prevents charge sharing effects fromaffecting voltages at the output node when switching between chargingand discharging the output node.
 18. The charge pump circuit defined inclaim 9 further comprising replica feedback circuitry that minimizescurrent mismatch between the first and second current sources.
 19. Amethod of adjusting a charge pump circuit comprising a current source,an output node, and a positive power supply voltage line on anintegrated circuit, the method comprising: loading data intoprogrammable elements on the integrated circuit; applying controlsignals from the loaded programmable elements to the charge pumpcircuit; and using a low-pass filter in a feedback path from the outputnode through the current source back to the output node to reduce noisefeedback along the feedback path, wherein the current source comprises atransistor and wherein the transistor is part of the feedback path,wherein the charge pump circuit further comprises a plurality ofparallel controllable transistor stages, wherein applying the staticcontrol signals comprises applying the static control signals to theplurality of parallel controllable transistor stages, and wherein eachof the parallel controllable transistor stages has a source-drainterminal that is directly connected to the positive power supply voltageline.
 20. The method defined in claim 19 further comprising reducingcurrent mismatch within the charge pump circuit.
 21. The method definedin claim 19 further comprising providing a static current contributionto the current source to prevent an output voltage from becoming stuckat a voltage level.
 22. A charge pump circuit comprising: a charge pumpcore having first and second current sources, an output node, and apositive power supply voltage line, wherein at least one of the currentsources comprises parallel adjustable transistor stages each connectedto the positive power supply voltage line, the charge pump corecomprising a transistor that provides a static current contribution tothe at least one current source to prevent an output voltage on theoutput node from becoming stuck at a voltage level; replica feedbackcircuitry coupled to the charge pump core that minimizes currentmismatch between the first and second current sources; and a low-passfilter connected between the charge pump core and the replica feedbackcircuitry that reduces noise feedback at the output node.
 23. A chargepump circuit comprising: a charge pump core having a first currentsource, an output node, and a positive power supply voltage line,wherein the first current source comprises parallel adjustabletransistor stages each having a source-drain terminal directly connectedto the positive power supply voltage line and wherein the first currentsource contains at least one transistor having a gate that is part of afeedback path from the output node through the first current source backto the output node; and a low pass filter having a capacitor connectedbetween the gate and a source of positive voltage and a resistorconnected to the gate in the feedback path.